Process for thermoforming infusion packets

ABSTRACT

Provided is a process for thermoforming a gas and liquid permeable layer ( 10 ) of thermoplastic material having an average thickness of less than 1.0 mm, the process comprising the steps of bringing the layer of thermoplastic material, at a temperature below that required for thermoforming, into contact with a mould ( 16 ) at a temperature above that of a thermoforming temperature of the thermoplastic material; pressing the mould into contact with the layer of thermoplastic material, the contact between mould and thermoplastic material causing heat to transfer from the mould to the thermoplastic material and raising the temperature of the thermoplastic material to a thermoformable temperature; such pressing thereby causing thermoforming of the thermoplastic material to conform to the shape of the mould.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the manufacture of packets, inparticular to infusion packets such as tea bags having a pre-determinedthree-dimensional shape.

BACKGROUND TO THE INVENTION

For many years infusion packets, such as tea bags were availableprimarily as square or round two-ply sheets of porous filter material,typically made of paper, with the infusible material, such as tea,sandwiched between the sheets. Such packets restrict the flow ofinfusible material within the packet substantially to two dimensions. Asa result the infusion performance of such packets is limited.

Thus the past few decades have seen the development of mass-producedinfusion packets which have a more three-dimensional shape and whichallow the infusible substance more room to move. Of particular successhave been the tetrahedral-shaped packets such as those described in theinternational patent applications published as WO 95/01907 (Unilever)and WO 2004/033303 (I.M.A. SPA).

In the manufacture of tetrahedral packets, the tetrahedral shape isconventionally formed by making mutually perpendicular transverse sealsin a tube of filter material and apparatus designed for such manufactureis ill-suited to the manufacture of other three-dimensional shapes.

Therefore, it would be desirable to develop a process which canmanufacture a variety of three-dimensional shapes.

DEFINITIONS

It should be noted that in specifying any range of values, anyparticular upper value can be associated with any particular lowervalue.

For the avoidance of doubt, the word “comprising” is intended to mean“including” but not necessarily “consisting of” or “composed of”. Inother words, the listed steps or options need not be exhaustive.

The disclosure of the invention as found herein is to be considered tocover all embodiments as found in the claims as being multiply dependentupon each other irrespective of the fact that claims may be foundwithout multiple dependency or redundancy.

SUMMARY OF THE INVENTION

The inventors have realised that known thermoforming processes, whilstcapable of generating a variety of three-dimensional shapes, are notsuitable for use with infusion packet material.

Firstly, infusion packets are most commonly made of paper, which is notthermoformable. Secondly, even if they were made from a thermoformablematerial, they would be inappropriate for thermoforming due to theirporosity and thinness of the material.

Known thermoforming processes involve a first step of heating thematerial followed by a second step of thermoforming the material. Thevery small heat capacity of infusion packet material means that anyheating will quickly be lost and so this approach will not work.

Even if a way of solving the heating problem could be found, theporosity of the material prevents the use of air pressure to form thematerial. Known thermoforming techniques typically involve the use ofair pressure to form the material. However, the porosity of infusionpacket material makes this approach impractical, as any difference inair pressure across the material will quickly equalise.

If air pressure is not used, and a mould was pressed into the materialthen only a limited number of three-dimensional shapes could be formeddue to the fragility of any porous and thin infusion packet material.

Thus, it would seem that thermoforming is not a practical method ofgenerating a wide variety of three-dimensional shapes from infusionpacket material.

However, surprisingly the present inventors have overcome these barriersand developed a thermoforming process that can produce three-dimensionalshapes from infusion packet material.

Thus, the invention relates to a process for thermoforming a gas andliquid permeable layer of thermoplastic material having an averagethickness of less than 1.0 mm, the process comprising the steps ofbringing the layer of thermoplastic material, at a temperature belowthat required for thermoforming, into contact with a mould at atemperature above that of a thermoforming temperature of thethermoplastic material; pressing the mould into contact with the layerof thermoplastic material, the contact between mould and thermoplasticmaterial causing heat to transfer from the mould to the thermoplasticmaterial and raising the temperature of the thermoplastic material to athermoformable temperature; such pressing thereby causing thermoformingof the thermoplastic material to conform to the shape of the mould.

Thus, no heating of the thermoplastic material is carried out untilthermoforming begins. As the thermoplastic material has such a lowcapacity to store heat, this means that it will rapidly heat up once itcomes into contact with the heated mould. Thus, the invention takes thedisadvantage of having a very low heat capacity and utilises thisfeature to develop a very effective thermoforming method for suchmaterials.

As the temperature of the thermoplastic material is only indirectlycontrolled due to heating from the mould, variations in the temperaturewill exist during thermoforming. Thus, it is preferably that thethermoplastic material exhibits plastic deformation behaviour over awide range of temperatures, preferably over a temperature range of atleast 20° C., more preferably over a temperature range of at least 40°C.

The thermoplastic material may be made from a wide variety of materials,however polyethylene terephthalate (PET) and poly lactic acid (PLA) arepreferred. Poly lactic acid (PLA) is particularly preferred due to itexhibiting plastic deformation over a wide range of temperatures.

In order for the process to be effective, the mould is preferablysignificantly hotter than the thermoplastic material. Thus, preferablythe temperature difference between the mould and the thermoplasticmaterial before coming into contact with the mould is greater than 40°C., preferably greater than 60° C., more preferably from 80° C. to 200°C.

In a preferred embodiment, the thermoplastic material is at roomtemperature prior to contact with the heated mould. Thus, thethermoplastic material before coming into contact with the mould is at atemperature of from 15° C. to 35° C.

As the primary application of the formed materials is as infusionpackets, typically the material will be very thin. Thus, preferably thethermoplastic material has an average thickness of less than 0.5 mm,more preferably less than 0.20 mm, most preferably from 0.01 to 0.10 mm.

The thermoplastic material may be made from a variety of designs, but ispreferably made from fibres of thermoplastic material, more particularlyfrom woven thermoplastic fibres. Such fibres may have a diameter of lessthan 0.5 mm, preferably less than 0.25 mm, more preferably less than0.10 mm, most preferably from 0.001 to 0.05 mm.

The temperature of thermoforming is sufficient to allow thethermoplastic material to deform under thermoforming stresses.Therefore, the temperature of the mould is preferably at least 100° C.,more preferably from 120° C. to 210° C. so that the resulting materialcan tolerate temperatures up to this level without shrinking back to itsoriginal sheet form.

The primary application of the formed materials is as infusion packets,therefore the thermoformed thermoplastic material is preferably gas andliquid permeable. In particular, it is preferred that the thermoformedthermoplastic material is permeable to aqueous liquids.

The process according to the present invention is capable of generatingthree-dimensional shapes which can then be used as infusion packetmaterial. For example, shapes such as tetrahedral, hemispherical and thelike are possible.

Thus, the process is generally followed by the step of depositing aparticulate product, typically comprising infusible entities such as tealeaves, into the thermoformed thermoplastic material. This step is thentypically followed by sealing the thermoformed material to produce asealed porous infusion packet.

The invention will now by illustrated by way of example and withreference to the following figures, in which:

FIG. 1 is a schematic representation of thermoforming apparatus suitablefor carrying out a process according to the present invention

FIG. 2 is a schematic representation of the same apparatus as shown inFIG. 1 during carrying out a process according to the present invention.

Turning to the figures, FIG. 1 shows a woven sheet of 50 micrometresthick poly lactic acid 10 clamped in place by upper clamps 12 and lowerclamps 14. The woven sheet 10 is at 20° C.

Positioned above the woven sheet is a male former 16, at a temperatureof 120° C.

In use, the male former 16 moves downwards to come into contact with thewoven sheet 10. As it makes contact, the portion of the woven sheet 10that come into contact with the male former 16 rapidly heat up to 120°C. due to its very low heat capacity.

The male former continues to move downwards, heating and thermoformingthe woven sheet simultaneously, until the male former is in the positionshown in FIG. 2.

Once the woven sheet 10 is fully thermoformed, the male former retractsand the woven sheet rapidly cools and sets, again due to its very lowheat capacity.

1. A process for thermoforming a gas and liquid permeable layer of thermoplastic material having an average thickness of less than 1.0 mm, the process comprising the steps of bringing the layer of thermoplastic material, at a temperature below that required for thermoforming, into contact with a mould at a temperature above that of a thermoforming temperature of the thermoplastic material; pressing the mould into contact with the layer of thermoplastic material, the contact between mould and thermoplastic material causing heat to transfer from the mould to the thermoplastic material and raising the temperature of the thermoplastic material to a thermoformable temperature; such pressing thereby causing thermoforming of the thermoplastic material to conform to the shape of the mould, followed by the step of depositing a particulate product into the thermoformed thermoplastic material.
 2. A process according to claim 1, wherein the thermoplastic material exhibits plastic deformation behaviour over a temperature range of at least 20° C., preferably over a temperature range of at least 40° C.
 3. A process according to claim 1, wherein the thermoplastic material comprises polyethylene terephthalate or poly lactic acid.
 4. A process according to claim 1, wherein the temperature difference between the mould and the thermoplastic material before coming into contact with the mould is greater than 40° C., preferably greater than 60° C., more preferably greater than 80° C.
 5. A process according to claim 1, wherein the thermoplastic material, before coming into contact with the mould, is at a temperature of from 15° C. to 35° C.
 6. A process according to claim 1, wherein the thermoplastic material has an average thickness of less than 0.5 mm, preferably less than 0,20 mm, more preferably less than 0.10 mm.
 7. A process according to claim 1, wherein the thermoplastic material is made from fibres of thermoplastic material.
 8. A process according to claim 7, wherein the fibres are woven thermoplastic fibres.
 9. A process according to claim 8, wherein the fibres have a diameter of less than 0.5 mm, preferably less than 0.25 mm, more preferably less than 0.10 mm, most preferably less than 0.05 mm.
 10. A process according to claim 1, wherein the temperature of the mould is at least 100° C., more preferably at least 120° C.
 11. A process according to claim 1, wherein the thermoplastic material is formed into a tetrahedral or hemispherical shape.
 12. A process according to claim 1, wherein the particulate product comprises infusible entities such as tea leaves.
 13. A process according to claim 12, which is followed by sealing the thermoformed material to produce a sealed porous infusion packet. 